A light-emitting element containing an organic compound as a luminous body, which has features such as thinness, lightness, high-speed response, and DC drive at a low voltage, is expected to be applied to a next-generation flat panel display or a next-generation lighting device. In particular, a display device in which light-emitting elements are arranged in matrix is considered to have advantages in a wide viewing angle and excellent visibility over a conventional liquid crystal display device.
It is said that, as for a light-emitting mechanism of a light-emitting element, an EL layer is interposed between a pair of electrodes and voltage is applied to the pair of electrodes, so that electrons injected from a cathode and holes injected from an anode are recombined in an emission center of the EL layer to form molecular excitons, and the molecular excitons release energy when returning to a ground state, whereby light is emitted. Singlet excitation and triplet excitation are known as excited states, and light emission can probably be achieved through either of the excited states.
An EL layer included in a light-emitting element includes at least a light-emitting layer. In addition, the EL layer can have a stacked-layer structure including a hole-injection layer, a hole-transport layer, an electron-transport layer, an electron-injection layer, and/or the like, in addition to the light-emitting layer.
As a material having semiconductor characteristics, metal oxide has attracted attention. The examples of such metal oxides having semiconductor characteristics are tungsten oxide, tin oxide, indium oxide, zinc oxide, and the like. A thin film transistor in which a channel formation region is formed using such metal oxide having semiconductor characteristics is known (for example, see Patent Documents 1 and 2).
A TFT applying an oxide semiconductor has high field effect mobility. Therefore, a driver circuit in a display device or the like can be formed using the TFT.